In the well-known Data Only Evolution of third generation CDMA based wireless communication systems, hereinafter referred to as 3G-1x EVDO, voice and data services are provided using separate frequency carriers. That is, the voice and data signals are transmitted over separate forward links defined by different frequency carriers. Data is transmitted over a time multiplexed frequency carrier at fixed data transmit powers but at variable data rates. Specifically, measured SIR at a receiver of a pilot signal transmitted by a base station is used to determine a data rate which can be supported by the receiver. Typically, the determined data rate corresponds to a maximum data rate at which a minimum level of quality of service can be achieved at the receiver. Higher measured SIR translates into higher data rates, wherein higher data rates involve higher order modulation and weaker coding than lower data rates. For example, if measured SIR at the receiver is 12 dB and −2 dB at two different receivers, then the data rates may be 2.4 Mb/s and 38.4 Kb/s at each of the respective receivers.
To improve system throughput, 3G-1x EVDO allows the receiver with the most favorable channel conditions, i.e., highest measured SIR, and thereby the highest associated data rate, to transmit ahead of receivers with comparatively less favorable channel conditions. 3G-1x EVDO utilizes a fast rate adaptation mechanism whereby the receiver, for every time slot, measures SIR, calculates a data rate using the measured SIR and reports the calculated data rate to the base station. Calculated data rates from multiple receivers are used by the base station to schedule when data transmission is to occur for a particular receiver.
Data transmission from the base station to a particular receiver occurs when that receiver reports the highest calculated data rate to the base station. The following protocol is utilized in data transmissions. The base station transmits data to the receiver in time slot n at the calculated data rate. The receiver receives the data transmission and responds with an ACK/NACK message indicating to the base station whether the data transmission was successfully received, i.e., no errors, by the receiver. Specifically, if the data transmission is successfully received, the receiver responds with an acknowledgement or ACK. Otherwise the receiver responds with a negative acknowledgement or NACK. The ACK/NACK message is received by base station in time slot n+j, wherein j is some known time offset. Thus, the base station can determine that an ACK/NACK message was transmitted from a receiver to which data was transmitted j time slots prior to receipt of the ACK/NACK message.
If an ACK was received, the base station knows that the data transmission to the associated receiver was successful. If a NACK was, the base station knows that the data transmission to the associated receiver was unsuccessful. In response to the NACK, the base station re-transmits, at the same data rate, the same data which was earlier transmitted. Note that the term “re-transmits the same data” should be understood to describe a retransmission of the data that may or may not be identical to the data it is being compared to, i.e., data transmitted in a previous transmission, so long as the data of the retransmission may be soft combined with the data to which it is being compared. The re-transmitted data is received by the receiver in time slot n+j+k, wherein k is some known time offset.
This prior art protocol disadvantageously utilizes the data rate of the initial transmission for re-transmissions even if the channel conditions may have changed for the associated receiver. Specifically, if the channel conditions degraded between the time of the initial transmission and the re-transmission, the re-transmission will likely suffer a higher frame error rate (FER) than the initial transmission, thereby suffering a degradation in transmission quality. Or if the channel conditions improved, then channel resources are being inefficiently utilized since a higher data rate could had been used for the re-transmission.